If you stay licensed, you have the option to enter the practice of law in the future with no administrative hassles.
In contrast, if you let your license lapse, you could have to study for and take the bar exam over again, go through an arduous background check known as a character and fitness review (reviewing your entire adult life), and face months of delay and significant expenditures of money to practice law again.
The character and fitness review also isn't just a time and money saver. Once you are admitted, you can only have your licensed revoked or suspended for doing something that violates professional ethics after you are admitted to practice. When you apply to the practice of law initially, you can be denied the right to practice law for bad characters for all manner of things in the broad discretion of the licensing officials. There are many kinds of conduct that can't be considered after you are admitted that can be used to deny you a license to practice law in the first place.
Effectively, you are doing the continuing education courses and paying the fees to keep the option of returning to the profession trouble free, open.
For example, I continue to be admitted to the practice of law in New York State, even though I no longer take cases there on a regular basis, so that in the one or two cases every few years that I do have in that state, I do not need to get a pro hac vice admission for a single case in that state, and so that if I ever want to relocate to New York State to practice law actively there, I can without a minimum of bureaucratic hurdles to doing so. Continuing education courses are also tax deductible in most cases.
Even if I was no longer actively practicing in either of the states where I am admitted to the practice of law, keeping the licenses in force would also allow me to apply for admission to the practice of law in many other states on a streamlined basis available for currently licensed lawyers who have been admitted to practice for at least five years.
Some states also allow you to go on "inactive" status with reduced fees and continuing education requirements, and then return to active practice again, by paying the fees and starting to take regular continuing education classes again, which makes this affordable during a period when you are exploring another career, considering an early retirement, or taking time off to raise kids.
In New York State, lawyers also have the ability to conduct certain kinds of real estate transactions for their clients without a real estate broker's license.
Once you are admitted to practice you are also grandfathered in if the requirements to be admitted to the practice of law change in the future.
For example, suppose New York State required a one year apprenticeship before you could practice independently starting in 2021. If you were admitted to practice in 2020, you would be exempt from that requirement.